The headlight according to the invention comprises a glass lens element and a support element made of plastics material, the two elements being associated at their periphery by matching assembly shapes of the interlocking type, fixed by an assembly glue which is preferably an epoxy resin or a plastisol.
The peripheral assembly shapes aforesaid can be edges, flanges, grooves, etc., and can be produced either in the lens element or in the support element of thermoplastic material.
The term "support element" as used in the present context means the element supporting the lens--i.e., either the reflector, or the extension tube, or the headlight casing.
It will be remembered that the reflector is the element of the headllight which is adapted to be equipped with at least one bulb co-operating optically with the reflector to send back the rays of light emitted by the bulb.
The extension tube means an intermediate, sleeve-shaped element, one of whose ends is attached to the reflector, the other end being attached to the lens. Such an element is used to lengthen the reflector, more particularly in the case of inclined lenses.
The casing means a receptacle to the opening of which the lens is attached and inside which a movable reflector is mounted, this kind of headlight enabling the height of the light beam to be adjusted by varying the inclination of the reflector.
With the development of plastics materials, support elements, more particularly reflectors, can now be moulded in thermoplastic material, for example, polyamide or polybutadiene terephthalate (PBT), such supports having the same properties as the conventional metal support elements. This is more particularly the case with thermoplastic reflectors, which have the same optical properties as the prior art metal reflectors.
The glue used for assembling the two elements sets as a result of curing at an appropriate temperature, of the order of 140.degree. to 150.degree. C.; this temperature must be lower than the softening temperature of the thermoplastic material under the influence of a slight stress. By way of example, the curing can be carried out at 140.degree. C. for a softening temperature under the influence of a slight stress of the thermoplastic material 150.degree. C.
The curing is generally performed by passing the two elements to be glued through a heating tunnel for about half an hour, the lens being disposed above the support element. On termination of the curing, the thermoplastic support element tends to flow as a result of its own weight and to become deformed and apply itself to the supporting surface offered by the lens.
The mass production of glass lenses prevents all the lenses from having a perfectly flat support surface. Consequently the support element may tend to become deformed, more particularly to become twisted, and this may have troublesome consequences for a reflector.
To obviate these disadvantages the Applicants propose to make the lens with internal supports cast in one piece with the lens and offering a true supporting plane. However, since at least four internal supports are required to provide an adequate bearing for the support element, it seems impossible to consistently make the four supports perfectly coplanar.
A first method which might be envisaged would be to provide three fixed internal supports defining a reference bearing plane and at least one deliberately "too long" extra support which would then be ground to the required size in relation to the dimensions of the lens. In practice it would suffice for such extra support to have a surplus dimension of 1.5 mm. Although this method is quite workable, it nevertheless has the disadvantage of requiring the grinding of the, or each extra support and the washing of the lens to get rid of the grinding dust.